ENGINEERING SYMBOLOGY, PRINTS, AND DRAWINGS Module 3 ... Textbooks/08... · ENGINEERING SYMBOLOGY, PRINTS, AND DRAWINGS Module 3 Electrical Diagrams and Schematics . ... Figure 11

ENGINEERING SYMBOLOGY, PRINTS,

AND DRAWINGS

Module 3

Electrical Diagrams and Schematics

Module 3: Electrical Diagrams and Schematics

i

TABLE OF CONTENTS

LIST OF FIGURES ......................................................................................................... ii

LIST OF TABLES .......................................................................................................... iii

REFERENCES .............................................................................................................. iv

OBJECTIVES ................................................................................................................ v

ELECTRICAL DIAGRAMS AND SCHEMATICS ............................................................ 1

Symbology ........................................................................................................... 2

Transformers ....................................................................................................... 2

Switches ............................................................................................................. 3

Fuses and Breakers ........................................................................................... 5

Relays, Contacts, Connectors, Lines, Resistors, and Miscellaneous Electrical Components ....................................................................................................... 6

Large Components ............................................................................................. 7

Types of Electrical Diagrams or Schematics ...................................................... 9

Reading Electrical Diagrams and Schematics .................................................. 14

Summary .......................................................................................................... 17

ELECTRICAL WIRING AND SCHEMATIC DIAGRAM READING EXAMPLES ................................................................................................. 18

Examples .......................................................................................................... 18

Summary .......................................................................................................... 23


ii

LIST OF FIGURES

Figure 1 Basic Transformer Symbols ............................................................................ 2

Figure 2 Transformer Polarity ........................................................................................ 2

Figure 3 Switches and Switch Symbols ......................................................................... 3

Figure 4 Switch and Switch Status Symbology ............................................................. 4

Figure 5 Fuse and Circuit Breaker Symbols .................................................................. 5

Figure 6 3-phase and Removable Breaker Symbols ..................................................... 6

Figure 7 Common Electrical Component Symbols ........................................................ 7

Figure 8 Large Common Electrical Components ........................................................... 8

Figure 9 Comparison of an Electrical Schematic and a Pictorial Diagram ..................... 9

Figure 10 Comparison of an Electrical Schematic and a Wiring Diagram .................... 10

Figure 11 Wiring Diagram of a Car's Electrical Circuit .................................................. 11

Figure 12 Schematic of a Car's Electrical Circuit .......................................................... 12

Figure 13 Example Electrical Single Line ..................................................................... 13

Figure 14 Examples of Relays and Relay Contacts ..................................................... 14

Figure 15 Ganged Switch Symbology .......................................................................... 15

Figure 16 Three-Phase Symbols .................................................................................. 16

Figure 17 Example 1 .................................................................................................... 19

Figure 18 Example 2 .................................................................................................... 20


iii

LIST OF TABLES

Table 1 Comparison Between Wiring and Schematic Diagrams ............................. 9


iv

REFERENCES

ASME Y14.5-2009, Dimensioning and Tolerancing.

IEEE Std 315-1975 (Reaffirmed 1993), Graphic Symbols for Electrical and

Electronic Diagrams.

Gasperini, Richard E., Digital Troubleshooting, Movonics Company; Los Altos,

California, 1976.

Jensen - Helsel, Engineering Drawing and Design, 7th Ed., McGraw-Hill Book

Company, New York, (August 15, 2007).

Lenk, John D., Handbook of Logic Circuits, Reston Publishing Company, Reston,

Virginia, 1972.

Wickes, William E., Logic Design with Integrated Circuits, John Wiley & Sons,

Inc, 1968.

Naval Auxiliary Machinery, United States Naval Institute, Annapolis, Maryland,

1951.

TPC Training Systems, Reading Schematics and Symbols, Technical Publishing

Company, Barrington, Illinois, 1974.

Arnell, Alvin, Standard Graphical Symbols, McGraw-Hill Book Company, 1963.

George Masche, Systems Summary of a Westinghouse Pressurized Water

Reactor, Westinghouse Electric Corporation, 1971.

Smith-Zappe, Valve Selection Handbook, 5th Ed., Gulf Publishing Company,

Houston, Texas, December 2003.


v

TERMINAL OBJECTIVE

1.0 Given an electrical print, READ and INTERPRET facility electrical diagrams and

schematics.

ENABLING OBJECTIVES

1.1 IDENTIFY the symbols used on engineering electrical drawings for the following

components:

a. Single-phase circuit breaker m. Electric motor

(open/closed) n. Meters

b. Three-phase circuit breaker o. Junctions

(open/closed) p. In-line fuses

c. Thermal overload q. Single switch

d. "a" contact r. Multiple-position switch

e. "b" contact s. Pushbutton switch

f. Time-delay contacts t. Limit switches

g. Relay u. Turbine-driven generator

h. Potential transformer v. Motor-generator set

i. Current transformer w. Generator (wye or delta)

j. Single-phase transformer x. Diesel-driven generator

k. Delta-wound transformer y. Battery

l. Wye-wound transformer

1.2 Given an electrical drawing of a circuit containing a transformer, DETERMINE

the direction of current flow, as shown by the transformer's symbol.

1.3 IDENTIFY the symbols and/or codes used on engineering electrical drawings to

depict the relationship between the following components:

a. Relay and its contacts

b. Switch and its contacts

c. Interlocking device and its interlocked equipment


vi

ENABLING OBJECTIVES (Cont.)

1.4 STATE the condition in which all electrical devices are shown, unless otherwise

noted on the diagram or schematic.

1.5 Given a simple electrical schematic and initial conditions, DETERMINE the

condition of the specified component (i.e., energized/de-energized, open/closed).

1.6 Given a simple electrical schematic and initial conditions, IDENTIFY the power

sources and/or loads and their status (i.e., energized or de-energized).


1

ELECTRICAL DIAGRAMS AND SCHEMATICS

To read and interpret electrical diagrams and schematics, the basic

symbols and conventions used in the drawing must be understood. This

chapter concentrates on how electrical components are represented on

diagrams and schematics. The function of the individual electrical

components and the theory behind their operation is covered in more

detail in the Electrical Science Handbook.

EO 1.1 IDENTIFY the symbols used on engineering electrical drawings for the

following components:

a. Single-phase circuit breaker m. Electric motor

(open/closed) n. Meters

b. Three-phase circuit breaker o. Junctions

(open/closed) p. In-line fuses

c. Thermal overload q. Single switch

d. "a" contact r. Multiple-position switch

e. "b" contact s. Pushbutton switch

f. Time-delay contacts t. Limit switches

g. Relay u. Turbine-driven generator

h. Potential transformer v. Motor-generator set

i. Current transformer w. Generator (wye or delta)

j. Single-phase transformer x. Diesel-driven generator

k. Delta-wound transformer y. Battery

l. Wye-wound transformer

EO 1.2 Given an electrical drawing of a circuit containing a transformer, DETERMINE

the direction of current flow, as shown by the transformer's symbol.

EO 1.3 IDENTIFY the symbols and/or codes used on engineering electrical drawings

to depict the relationship between the following components:

a. Relay and its contacts

b. Switch and its contacts

c. Interlocking device and its interlocked equipment

EO 1.4 STATE the condition in which all electrical devices are shown, unless

otherwise noted on the diagram or schematic.

EO 1.5 Given a simple electrical schematic and initial conditions, DETERMINE the

condition of the specified component (i.e., energized/de-energized,

open/closed).


2

Figure 1 Basic Transformer Symbols

Symbology

To read and interpret electrical diagrams and schematics, the reader must first be well versed in what the many symbols represent. This chapter discusses the common symbols used to depict the many components in electrical systems. Once mastered, this knowledge should enable the reader to successfully understand most electrical diagrams and schematics.

The information that follows provides details on the basic symbols used to represent components in electrical transmission, switching, control, and protection diagrams and schematics.

Transformers

The basic symbols for the various types of transformers are shown in Figure 1 (A). Figure 1 (B) shows how the basic symbol for the transformer is modified to represent specific types and transformer applications.

In addition to the transformer symbol itself, polarity marks are sometimes used to indicate current flow in the circuit. This information can be used to determine the phase relationship (polarity) between the input and output terminals of a transformer. The marks usually appear as dots on a transformer symbol, as shown in Figure 2.

Figure 2 Transformer Polarity


3

On the primary side of the transformer the dot indicates current in; on the secondary

side the dot indicates current out.

If at a given instant the current is flowing into the transformer at the dotted end of the

primary coil, it will be flowing out of the transformer at the dotted end of the secondary

coil. The current flow for a transformer using the dot symbology is illustrated in Figure 2.

Switches

Figure 3 shows the most common types of switches and their symbols. The term "pole,"

as used to describe the switches in Figure 3, refers to the number of points at which

current can enter a switch. Single pole and double pole switches are shown, but a

switch may have as many poles as it requires to perform its function. The term "throw"

used in Figure 3 refers to the number of circuits that each pole of a switch can complete

or control.

Figure 3 Switches and Switch Symbols


4

Figure 4 provides the common symbols that are used to denote automatic switches and

explains how the symbol indicates switch status or actuation.

Figure 4 Switch and Switch Status Symbology


5

Fuses and Breakers

Figure 5 depicts basic fuse and circuit breaker symbols for single-phase applications. In

addition to the graphic symbol, most drawings will also provide the rating of the fuse

next to the symbol. The rating is usually in amps.

Figure 5 Fuse and Circuit Breaker Symbols


6

When fuses, breakers, or switches are used in three-phase systems, the three-phase

symbol combines the single-phase symbol in triplicate as shown in Figure 6. Also

shown is the symbol for a removable breaker, which is a standard breaker symbol

placed between a set of chevrons. The chevrons represent the point at which the

breaker disconnects from the circuit when removed.

Figure 6 Three-phase and Removable Breaker Symbols

Relays, Contacts, Connectors, Lines, Resistors,

and Miscellaneous Electrical Components

Figure 7 shows the common symbols for relays, contacts, connectors, lines, resistors,

and other miscellaneous electrical components.


7

Figure 7 Common Electrical Component Symbols

Large Components

The symbols in Figure 8 are used to identify the larger components that may be found in

an electrical diagram or schematic. The detail used for these symbols will vary when

used in system diagrams. Usually the amount of detail will reflect the relative

importance of a component to the particular diagram.


8

Figure 8 Large Common Electrical Components


9

Types of Electrical Diagrams or Schematics

There are three ways to show electrical circuits. They are wiring, schematic, and

pictorial diagrams. The two most commonly used are the wiring diagram and the

schematic diagram. The uses of these two types of diagrams are compared in Table 1.

TABLE 1

Comparison Between Wiring and Schematic Diagrams

Wiring Diagrams Schematic Diagrams

1. Emphasize connections between elements of a circuit or system

1. Emphasize "flow" of system

2. Use horizontal and vertical lines to represent the wires

2. Use horizontal and vertical lines to show system flow

3. Use simplified pictorials that clearly resemble circuit/system components

3. Use symbols that indicate function of equipment, but the symbols do not look like the actual equipment

4. Place equipment and wiring on drawing to approximate actual physical location in real circuit

4. Drawing layout is done to show the "flow" of the system as it functions, not the physical layout of the equipment

The pictorial diagram is usually

not found in engineering

applications for the reasons

shown in the following

example. Figure 9 provides a

simple example of how a

schematic diagram compares

to a pictorial equivalent. As can

be seen, the pictorial version is

not nearly as useful as the

schematic, especially if you

were trying to obtain enough

information to repair a circuit or

determine how it operates.

Figure 9 Comparison of an Electrical Schematic and a

Pictorial Diagram


10

Figure 10 provides an example of the relationship between a schematic diagram (Figure

10A) and a wiring diagram (Figure 10B) for an air drying unit. A more complex example,

the electrical circuit of an automobile, is shown in wiring diagram format in Figure 11

and in schematic format in Figure 12. Notice that the wiring diagram (Figure 11) uses

both pictorial representations and schematic symbols. The schematic (Figure 12) drops

all pictorial representations and depicts the electrical system only in symbols.

Figure 10 Comparison of an Electrical Schematic and a Wiring Diagram


11

Figure 11 Wiring Diagram of a Car's Electrical Circuit


12

Figure 12 Schematic of a Car's Electrical Circuit

When dealing with a large power distribution system, a special type of schematic

diagram called an electrical single line is used to show all or part of the system. This

type of diagram depicts the major power sources, breakers, loads, and protective

devices, thereby providing a useful overall view of the flow of power in a large electrical

power distribution system.

On power distribution single lines, even if it is a 3-phase system, each load is commonly

represented by only a simple circle with a description of the load and its power rating

(running power consumption). Unless otherwise stated, the common units are kilowatts

(kW). Figure 13 shows a portion of an electrical distribution system at a nuclear power

plant.


13

Figure 13 Example Electrical Single Line


14

Figure 14 Examples of Relays and Relay Contacts

Reading Electrical Diagrams and Schematics

To read electrical system diagrams and schematics properly, the condition or state of

each component must first be understood. For electrical schematics that detail

individual relays and contacts, the components are always shown in the de-energized

condition (also called the shelf-state).

To associate the proper relay with the contact(s) that it operates, each relay is assigned

a specific number and/or letter combination. The number/letter code for each relay is

carried by all associated contacts. Figure 14 (A) shows a simple schematic containing a

coil (M1) and its contact. If space permits, the relationship may be emphasized by

drawing a dashed line (symbolizing a mechanical connection) between the relay and its

contact(s) or a dashed box around them as shown in Figure 14 (B). Figure 14 (C)

illustrates a switch and a second set of contacts that are operated by the switch.


15

When a switch is used in a circuit, it may contain several sets of contacts or small

switches internal to it. The internal switches are shown individually on a schematic. In

many cases, the position of one internal switch will affect the position of another. Such

switches are called ganged switches and are symbolized by connecting them with a

dashed line as shown in Figure 15 (A). In that example, closing Switch 1 also closes

Switch 2. The dashed line is also used to indicate a mechanical interlock between two

circuit components. Figure 15 (B) shows two breakers with an interlock between them.

Figure 15 Ganged Switch Symbology

In system single line diagrams, transformers are often represented by the symbol for a

single-phase air core transformer; however, that does not necessarily mean that the

transformer has an air core or that it is single phase. Single line system diagrams are

intended to convey only general functional information, similar to the type of information

presented on a P&ID for a piping system. The reader must investigate further if more

detail is required. In diagrams depicting three-phase systems, a small symbol may be


16

placed to the side of the transformer primary and secondary to indicate the type of

transformer windings that are used.

Figure 16 (A) shows the most commonly used symbols to indicate how the phases are

connected in three-phase windings. Figure 16 (B) illustrates examples of how these

symbols appear in a three-phase single line diagram.

Figure 16 Three-Phase Symbols


17

Summary

The important information in this chapter is summarized below.

Electrical Diagrams and Schematics Summary

This chapter covered the common symbols used on electrical diagrams and

schematics to represent the basic electrical components.

Polarity on a transformer is defined by dots placed on the primary and

secondary windings. On the primary side, the dot indicates current in; on the

secondary, the dot indicates current out.

Switches, relays, and interlocked equipment commonly use dashed lines or

boxes to indicate the relationship between them and other components.

Electrical components, such as relays, are drawn in the de-energized state

unless otherwise noted on the diagram.


18

ELECTRICAL WIRING AND SCHEMATIC DIAGRAM

READING EXAMPLES

This chapter contains several examples that will help to build, through

practice, on the knowledge gained in reading electrical wiring and

schematic diagrams.

1.6 Given a simple electrical schematic and initial conditions, IDENTIFY the

power sources and/or loads and their status (i.e., energized or de-

energized).

Examples

To aid in understanding the symbology and diagrams discussed in this module refer to

Figure 17 and Figure 18. Then answer the questions asked about each. The answers

for each example are given on the page following the questions.

Referring to Figure 17:

1. What type of diagram is it?

2. What is the rating on the fuses protecting the motor controller circuit?

Refer to the number at the far left to locate the following lines.

3. What is the component labeled ITDR in line 13?

4. Which lines contain limit switches?

5. Which lines contain pushbutton switches?

6. How many contacts are operated from relay 8CR?

7. What component is represented by the symbol on the far right of line 4?


19

Figure 17 Example 1


20

Answers to questions on Figure 17.

1. Schematic

2. 10 amps

3. A time delay closing switch

4. Lines 7, 9, 11, 12, 14, and 15

5. Lines 3, 4, 5, 6, and 18

6. 4.

7. A green lamp

Figure 18 Example 2


21

Referring to Figure 18.

1. What type of diagram is Figure 18?

2. How many current transformers are in the diagram?

3. What type of circuit breakers are shown?

4. What is the voltage on the main bus?

5. What is the voltage entering the transformer in the lower left corner?

6. Classify the transformer in the upper left corner.

7. What is the component in the lower left corner?


22

Answers to questions on Figure 18.

1. System diagram

2. 3. If you said 4, the one in the upper right is a potential transformer.

3. Drawout type.

4. 4.16 kV or 4160 V.

5. 480 V.

6. Delta primary, grounded wye secondary.

7. (Emergency) diesel generator


23

Summary

The important information in this chapter is summarized below.

Electrical Wiring and Schematic Diagram Reading Example Summary

This chapter reviewed the material presented in this module through the practice

reading examples.

Documents

ENGINEERING SYMBOLOGY, PRINTS, AND DRAWINGS Module 3 ... Textbooks/08... · ENGINEERING SYMBOLOGY, PRINTS, AND DRAWINGS Module 3 Electrical Diagrams and Schematics . ... Figure 11